The present invention relates to an installation for controlling tooling that includes a clamp for the purpose of performing a particular operation on workpieces, relative displacements between the clamp and said workpieces being controlled by an automatic positioning system, and more particularly to such an installation including a unit for controlling said operation, a servo-control unit suitable for controlling mainly the opening and closing of said clamp via drive means, a unit for controlling said positioning system, and a sensor suitable for providing information relating to progress of said operation.
The tooling having a clamp or other clamping members may be constituted, for example, by an electric spot welding clamp, by a tool for clamping or crimping or stapling, or by any other tool of the same type in which two co-operating members of the hinged or guided jaw type are required at a given moment to move towards the workpiece(s) to be treated or assembled, and then to move apart, and to be transferred to another work position. The automatic positioning system may be constituted by a robot or by a manipulator, under the control of a programmable controller or computer, with control being numerical or the like.
Mention is made above of relative displacement between the clamp and the workpieces. That covers the robot moving the workpieces, while the tooling including the clamp is stationary relative to the ground, and also the robot transporting the tooling, while the workpiece(s) is/are stationary relative to the ground.
The above-mentioned drive means that are servo-controlled in position may be constituted by a servo-controlled pneumatic actuator, by a servo-controlled hydraulic actuator, or by a servo-controlled electric motor associated with a mechanical system.
The electric motor technique has been described, in particular, in the following patent applications filed in the name of the Applicant:
French patent application No. 85 12239 filed Aug. 9, 1985;
European patent application No. 87 400305 filed Feb. 11, 1987; and
French patent application No. 89 12344 filed Sep. 20, 1989.
To describe the problem that the present invention seeks to solve, there follows a description of the two techniques presently known in the type of installation defined above, assuming that the problem is to control a resistance welding clamp fitted with a servo-controlled electrical actuator.
FIG. 1 shows a welding clamp fitted with an electrical actuator 1. The clamp is said to be an "X-clamp" since it includes a single hinge at 7. The clamp is constituted by an upper electrode-carrying arm 3 and a lower electrode-carrying arm 2. These two arms are hinged relative to each other about the above-mentioned hinge axis 7. One of the two arms may be connected to the fixed cradle 14 of the clamp by a backing-off and automatic centering system such as that described in French patent application No. 89 12344 filed Sep. 20, 1989. The electrical actuator 1 is connected firstly to the arm 3 via an actuator rod 11, and secondly to the arm 2 via an actuator stator 5 and a crank 2a. The electric actuator is constituted by an electric motor whose rotor 10 receives the nut of a nut and ballscrew system 6 for transforming the rotary motion of the motor into translation of the rod 11. The motor includes a position sensor 8 of the resolver type or of some other type (incremental, potentiometer, etc.), thereby enabling the control system to be informed about the angular position of the rotor and thus enabling the motor and ballscrew assembly to be servo-controlled in position and in velocity. Other sensors specific to the application (for measuring force, current, acceleration, etc.) may also be installed.
In the example described, the clamp is fixed on the wrist 9 of a robot suitable for displacing the clamp, with the workpieces 12' and 13' to be welded remaining stationary relative to the ground.
Together the motor and the hinge make it possible to close the clamp, i.e. to clamp sheet metal workpieces 12' and 13' between the electrodes 2' and 3', thereby bringing the workpieces towards each other. Once sufficient force is applied, current is passed through the assembled workpieces so as to form a melt spot between the workpieces 12' and 13', thereby obtaining a spot weld N. Once the welding cycle is completed, the motor is controlled to open the arms 2 and 3, with the servo-control system opening them far enough apart to enable them to reach the following spot N+1. Once the clamp has been opened, the robot can displace it to the following spot and start a new welding cycle.
FIGS. 2a and 2b show a clamp 12 mounted at the end of a robot 13 under the control of a control unit 14. The clamp is connected to a welding cabinet 15 (the unit that controls the operation) for controlling the welding cycle, i.e. for monitoring the flow of welding current through the workpieces to be welded.
Two configurations may be provided for the unit 16 that servo-controls the clamp motor:
Configuration No. 1: This configuration is shown in FIG. 2a where the servo-control unit 16 is included in the welding cabinet 15, thereby presenting numerous advantages:
a) accurate synchronization can be obtained between the position of the electrodes, the force between the electrodes, and the welding current. This gives rise to a reduction in cycle time (welding current can be applied as soon as the necessary force is achieved);
b) improved quality (while the current is flowing, the strength of the force is known reliably);
c) improved reliability (when the clamp closes, numerous tests relating to the distance between the electrodes can be performed: whether the thickness of the stack of sheet metal is correct; how far the electrodes have worn, etc.); and
d) all of the parameters relating to welding are brought together in a single control unit, thereby providing a clear functional separation between the welding portions and the handling portions (robot).
Unfortunately, this configuration suffers from a drawback due to the fact that the opening of the clamp is not controlled by the robot: before the robot can be allowed to move to the following spot, the clamp must have opened sufficiently to ensure that any obstacles between the two spots can be cleared. This gives rise to the trajectory shown in FIG. 3a. Unfortunately, the time required to obtain such opening (portion a-b) is added to the cycle time, thereby penalizing the productivity of the installation. Similarly, closure can be initiated only once the robot has reached the next spot (spot c). The time required for closure (c-d) is then added to the displacement time of the robot.
Configuration No. 2: This configuration corresponds to FIG. 2b in which the servo-control unit 16 is integrated in the robot control cabinet 14. The cabinet 14 can then control the clamp as though it constituted an additional axis (e.g. a seventh axis if the robot already has six), and as a result it can synchronize clamp opening with the other displacements of the robot. This gives rise to the trajectory shown in FIG. 3b. This technique has an immediate advantage in that the time required for opening and closing the clamp on going from one spot to another can be masked completely or in part by the displacement time of the robot, thereby giving rise to significant productivity gain. In addition, this concept greatly facilitates on-site programming by robot training.
In contrast, this method includes as drawbacks the "advantages" listed for the preceding technique:
a) difficulty of obtaining synchronization between the position of the electrodes, the force between the electrodes, and the welding current, thereby lengthening cycle times;
b) this synchronization difficulty can give rise to reduced quality;
c) no specialized test functions relating to the welding operation: monitoring the thickness of the stack of sheet metal workpieces, monitoring the extent of electrode wear, etc.;
d) no clear separation between welding functions and handling functions (robot), thereby complicating adjustment, development, and maintenance; and
e) it is sometimes necessary to perform special software development in the control cabinet for the robot or the manipulator, for example to make it possible to provide position and force servo-control for the tool.